1. Field of Invention
This invention relates to carbonless record materials. It more particularly relates to pressure-sensitive record materials in the form of self-contained carbonless record sheets.
Self-contained pressure-sensitive record materials is a type of pressure-sensitive manifold sheet, usually paper, wherein a color-forming reaction is used between an electron donating chromogenic material (color former) and an electron accepting coreactant (developer). The record sheet can be formed by coating a substrate with a layer of microcapsules enclosing an oily solution having a color former dissolved or dispersed therein and with another layer of a developer. Usually the acidic developer layer is formed over the color former microcapsule layer. An alternate construction is to coat a substrate with a mixture of microcapsules and acidic developer in a single layer interspersed on the same substrate, which can be accomplished in a single coating step.
2. Description of Related Art
Self-contained record materials are described in general, for example, in U.S. Pat. Nos. 4,197,346; 4,486,762 and 4,680,597. Also in United Kingdom Pat. No. 1,215,618. U.S. Pat. No. 4,197,346 describes self-contained pressure-sensitive record materials comprising an oil-soluble phenol-formaldehyde novolak resin or an oil-soluble metal salt of a phenol-formaldehyde novolak resin as a color-developer. British Pat. No. 1,215,618 teaches self-contained record materials containing a solution of chromogenic material and a top coating comprising a mixture of kaolin clay and an oil-soluble phenol-formaldehyde novolak resin.
Various configurations comprising a self-contained, single coating of color-forming reactant material have been proposed in the prior art. U.S. Pat. No. 3,576,660 discloses a pressure-sensitive record sheet having encapsulated color-forming co-reactants in a single coating layer. In this system, one group of capsules contains an oily solution of a chromogenic material and a second group of capsules contains an oily solution of phenolic polymeric material. Special capsule wall materials are taught used to help solve the problem of premature coloration caused by the close proximity of the color-forming reactants in the single layer.
Each of U.S. Pat. Nos. 2,929,736; 3,481,759 and 3,672,935 discloses a coated mixture of encapsulated chromogenic materials and unencapsulated solid particles. The chromogenic materials and the solid particles react with each other to form a colored mark when they are brought into contact. More specifically, U.S. Pat. No. 2,929,736 discloses a heat and pressure-sensitive record sheet having an encapsulated liquid chromogenic solution, solid co-reactive clay praticles and a polymeric film-forming protective material, all mixed together to yield a single, color-forming coating layer. U.S. Pat. No. 3,481,759 discloses single-coating, self-contained pressure-sensitive record material having a color-forming layer of capsules containing a liquid solution of color reactant, polymeric binder material and metal resinate coreactant particles. U.S. Pat. No. 3,672,935 teaches several forms of pressure-sensitive record material including self-contained sheets wherein an encapsulated liquid solution of one co-reactant and solid particles of another co-reactant can be combined in a single coating layer. In one embodiment, capsules of an oily solution of co-reactant are combined with unencapsulated solid particles of chromogenic compounds and, in another embodiment, capsules of oily solvent are combined with unencapsulated solid particles of chromogenic compounds and coreactant.
A drawback of self-contained papers is that imaging capability continues essentially unaltered after the first use. Subsequent in time imaging is not readily detectable as having occurred after the first imaging or marking. Self-contained papers have found little usefulness as security documents where tamper resistance is important.
More recently pressure-sensitive copying sheets principally transfer sheets using radiation curable coatings have been disclosed by such patents as U.S. Pat. Nos. 4,091,122; 4,110,511; 4,137,094 and 4,751,165. Photosensitive microcapsules and systems utilizing them are disclosed in U.S. Pat. Nos. 4,587,194 and 4,600,678. Transfer systems using photosensitive microcapsules have been marketed as the Cycolor process. The process uses a sandwich configuration and involves a donar substrate coated with three kinds of microcapsules, each sensitive to one of the primary colors. In response to projection of an image the capsules harden proportionate to the exposure to the appropriate color of light. The exposed substrate in contact with a developer ply is run through pressure rollers. Capsules not completely hardened are ruptured thereby to produce a color imgage on the receiver developer ply.
It is an object of the present invention to disclose a self-contained record sheet having improved tamper resistance by making subsequent-in-time imaging or marking detectable as such.